CHRONICLE

By SUSAN HELLER ANDERSON

Published: December 29, 1990

Horology, the science of measuring time, is a hobby of Dr. WILLIAM F. BAKER . So the president of Channel 13/WNET in New York will step in front of the camera Monday to explain why one second is being added to the world's official clocks. He'll apppear for precisely two minutes at precisely 23:58 Greenwich mean time, or 6:58 P.M. Eastern Standard Time.

The United States Naval Observatory, whose atomic clock is accurate to a billionth of a second a day, has calculated that the earth's rotation is slowing slightly. To keep the atomic time standard that we now live by synchronized with earth time, a "leap second" will be added, Dr. Baker said.

In 1972, it was decided by international accord that atomic clocks could run independently of the earth, while keeping the two time measurements within one second of each other. A leap second is added when necessary -- this is the 16th since 1972. "If the earth were to speed up, a leap second would be removed," Dr. Baker said.